1. Field of the Invention
The present invention relates to an optical pulse time spreading apparatus for use in optical multiplex transmission, and, more in particular, to an optical pulse time spreading apparatus utilizing a super-structured fiber Bragg grating (SSFBG) that consists of a plurality of unit diffraction gratings of periodic refractive index distribution structure disposed in the wave-propagating direction of an optical fiber.
2. Description of the Background Art
In recent years, the demand for telecommunications has rapidly increased as a result of, e.g. the spread of the Internet. Aiming at satisfying the increasing demand, high-capacity and high-speed telecommunications networks using optical fiber are now being completed. Furthermore, in order to establish high-capacity communications, a high value has been placed on optical multiplexing technology that transmits plural channels of optical pulse signals multiplexed on a single optical fiber transmission line.
As optical multiplexing technology, optical time division multiplexing (OTDM), wavelength division multiplexing (WDM) and optical code division multiplexing (OCDM) have been intensively researched.
Among those optical multiplexing schemes, the OCDM has the merit of flexibility on the operation side in that no restrictions are imposed on the time axis allocation to the respective bits of optical pulse signals that are transmitted and received. Furthermore, the OCDM is advantageous in that on the time axis a plurality of channels can be established on the same time slot while on the wavelength axis a plurality of communication channels can be established on the same wavelength.
In the context, the words “optical pulse signal” refer to an optical pulse train conveying a digital signal having a binary value. More specifically, an optical pulse signal refers to a train or stream of optical pulses which appear on the time axis at regular intervals corresponding to time intervals defined by the reciprocal of a frequency corresponding to a bit rate and represent a digital signal having binary values by the existence or nonexistence of optical pulses in the optical pulse train.
The OCDM is a telecommunications scheme in which codes that are different from channel to channel are allocated to channels as a pattern to extract signals by means of pattern matching. Such a telecommunications scheme is disclosed in Hideyuki Sotobayashi, “Optical Code Division Multiplexing Network”, Japanese Journal of Applied Physics, Vol. 71, No. 7, 2002, pp. 853-859, published by The Japan Society of Applied Physics. The OCDM is a solution for optical multiplexing in which an optical pulse signal is encoded on the transmitter side with an optical code that is different from communication channel to channel and the pulse signal is decoded on the receiver side with the same codes as on the transmitter side to restore the original optical pulse signal.
Since, during decoding, only the optical pulse signals whose codes correspond to ones used for encoding are extracted and processed as effective signals, it is possible to allocate an optical pulse signal of the same wavelength or a plurality of wavelengths combined to a plurality of communication channels. Furthermore, in order to carry out the decoding on the receiver side, the OCDM requires using the same codes as used in the encoding. Thus in the OCDM, the decoding side cannot execute decoding unless it knows the codes used for encoding. That ensures the security on data transmission.
The OCDM can multiplex a plurality of channels at the same time and on the same wavelength. In comparison with the OTDM and WDM, the OCDM can remarkably increase the communication capacity, so that the attention has been focused on establishing large-capacity data communications.
As means for encoding and decoding an optical pulse signal, passive optical devices, e.g. SSFBG and arrayed waveguide grating (AWG), are applicable which do not consume electric power. Since the passive optical devices can operate without incurring limitations on electrical processing speed, a communication apparatus utilizing a passive optical device can facilitate the increase in communication rate.
As a specific telecommunications scheme employing the OCDM technology, an OCDM communication scheme using binary phase codes has been known. Such an OCDM communication scheme is taught in Akihiko Nishiki, et al., “Development of Encoder/Decoder for OCDM using an SSFBG”, Technical Report of IEICE (The Institute of Electronics, Information and Communication Engineers), OFT2002-66, (2002-11). In addition, an OCDM communication employing multilevel phase codes which use multilevel phases has lately been studied. Such an OCDM communication is taught in P.C. Teh, et al., “Demonstration of a Four-channel WDM/OCDMA System Using 255-chip 320-Gchip/s Quarternary Phase Coding Gratings”, IEEE (Institute of Electrical and Electronics Engineers) Photonics Technology Letters, Vol. 14, No. 2, February 2002, and in Gabriella Cincotti, “Full Optical Encoders/Decoders for Photonic IP Routers”, Journal of Lightwave Technology, Vol. 22, No. 2, pp. 337-342, February 2004.
Hereinafter, a binary phase code or multilevel phase code will simply be referred to as phase code. An OCDM communication scheme using phase codes establishes communication through the following steps. Firstly, a transmitter according to the OCDM communication scheme uses an output from a multiple-wavelength continuous wave light source to produce a series, or train, of optical pulses, and then converts, or modulates, the optical pulse train with a transmitting signal, i.e. digital signal to be transmitted having binary values, to produce an optical pulse signal in the RZ (Return to Zero) format to be sent out. The optical pulse signal in an RZ format will simply be called as optical pulse signal in the following description.
The transmitter encodes the optical pulse signal to be transmitted by its encoder into an encoded optical pulse signal, and then transmits the latter. On the receiver side, a receiver receives the encoded optical pulse signal and decodes the encoded signal by means of a decoder, in which the same codes as in the encoder of the transmitter is set. Consequently, the transmitted optical pulse signal is reproduced.
In accordance with the OCDM communication scheme using phase codes, an optical pulse signal is spread in time on the time axis by an encoder according to a encoding rule set in the encoder into an encoded optical pulse signal. The above rule set in the encoder is defined by codes. Hereinafter, an encoded optical pulse of an optical pulse signal resultant from time-spreading on the time axis will be referred to as a chip pulse. In other words, the encoder has a function of spreading each optical pulse contained in the optical pulse signal into a train of chip pulses that are present on the time axis.
The encoded optical pulse signal is decoded to the original optical pulse signal by a decoder. The decoder restores each optical pulse from the chip pulse train in the encoded optical pulse signal to thereby decode the original optical pulse signal. Note that, if the transmitter generates an optical pulse signal to be transmitted by using a multiple-wavelength continuous wave light source, the obtained chip pulses are single-wavelength optical pulses whereas the individual optical pulses in the optical pulse signal contain light components of plural wavelengths.
In the relationship between an encoder and a decoder each utilizing SSFBGs, the codes set in the SSFBGs are the same as one another, but the settings in the input and output ends of the SSFBGs are reversed. More specifically, by referring to one and the other end of each SSFBG as A and B, respectively, in the encoder, each SSFBG has its input end A and its output end B while in the decoder, each SSFBG has its input end B and its output end A. Hence, in the encoder, the end part A of the SSFBGs is supplied with an optical pulse signal while the end part B outputs an encoded optical pulse signal, whereas in the decoder the end part B of the SSFBG is fed with the encoded optical pulse signal while the end part A outputs a decoded optical pulse signal.
As described above, the encoder and decoder using the SSFBGs are reversed as to the settings in the input and output ends, but are identical as to the devices per se, to each other. Therefore, either of the encoder and decoder may be called as an optical pulse time spreader in the following description.
The term “fiber Bragg grating (FBG)” specifically refers to an optical fiber having its core forming Bragg diffraction gratings having the refractive index thereof periodically modulated so that an optical signal having multiple wavelengths impinging thereon is rendered periodically intensified and weakened in refractive index. The FBG can therefore achieve a periodic refractive index modulation in the longitudinal direction of the optical fiber, and has a filtering function of reflecting light having a specific wavelength corresponding to a predetermined period while passing optical signals having the remaining wavelengths without detecting the periodic changes in refractive index. Such an FBG is taught in Naoya Wada, et al., “A 10 Gb/s Optical Code Division Multiplexing Using 8-Chip Optical Bipolar Code and Coherent Detection”, Journal of Lightwave Technology, Vol. 17, No. 10, October 1999. The FBG has a unique feature in that the refractive index of the core of an optical fiber is periodically modulated, but is the same in geometric form as an optical fiber for use in an optical transmission path in an optical communications system by ODDM. Hence, if the FBGs are used as a constituent element in an optical communication apparatus, the connection between the FBGs and the optical transmission path is made by connecting optical fibers. The connection between optical fibers can be attained much easier than connecting an optical fiber to a type of optical transmission path, such as a planer lightwave circuit (PLC), other than optical fiber.
The optical pulse time spreader uses an SSFBG formed by serially disposing a plurality of unit FBGs each having a certain length at a certain spatial period in the lengthwise direction of the core of an optical fiber. The unit FBG means a series of consecutive FBGs having no in between portions which change in modulation period or abruptly shift in phase of the refractive index.
The SSFBG for use in the encoder or decoder has phase shift parts provided between the neighboring unit FBGs. The amount of phase shift set in the phase shift parts is determined in dependent upon the codes set in the encoder or decoder. For example, if the SSFBG has a plurality (t) of unit FBGs, it has (t−1) phase shift parts formed, where t is a natural number more than unity. Consequently, the codes set in the SSFBG are determined depending on the amount of phase shifts set in those (t−1) positions.
As taught in the aforementioned reference, Wada et al., an encoder or decoder can employ, other than the SSFBG, a PLC having transversal filter configuration. Alternatively, AWG can be used. As an example of using AWG, there is a reference literature, Jing Cao et al., “Spectral Encoding and Decoding of Monolithic InP OCDMA Encoder”, Paper We. 3.6.6, Vol. 3, ECOC 2005. The encoders and decoders utilizing PLC or AWG have the feature that they are free from limitation on setting codes. However, the encoders and decoders using PLC or AWG are disadvantageous over the ones using SSFBG in that light loss is large and downsizing of devices is difficult.
In the following, the principle of encoding and decoding carried out by SSFBGs having the same codes will be described with reference to the accompanying drawings. An SSFBG encoder and an SSFBG decoder employ SSFBGs. In FIG. 1, each unit FBG of the SSFBGs is indicated by a rectangle with a pattern of lateral stripes. In addition, relative phases set in the unit FBGs are depicted with the numerical values presented beside the corresponding unit FBGs.
Now, arbitrary one of the unit FBGs, the constituent elements of the SSFBGs, is taken as a reference unit FBG. The phase of Bragg-reflected light output from the reference unit FBG is defined to zero. Furthermore, when a difference in phase of the Bragg-reflected light from another unit FBG with respect to the reference unit FBG is expressed as the phase of the former unit FBG, the phase of the reflected light from the reference unit FBG is referred to as a relative phase. Under those definitions, the phase of the Bragg-reflected light from each unit FBG in the SSFBGs is determined as a phase difference from the reference unit FBG. In order to describe the operation of SSFBGs that constitute optical pulse time spreaders, with respect to the phases of the Bragg-reflected light from the unit FBGs, only the mutual phase differences between the reflected lights output from the respective unit FBGs are critical.
In FIG. 1, the relative phase 0's (zeros) indicated beside some unit FBGs mean that the phases of the Bragg-reflected light output from the unit FBGs are equal to one another. Likewise, the phases of the reflected light from the unit FBGs represented by the relative phases 0.5 are equal to each other. The phase difference between the phase 0 of the reflected light from the unit FBGs and the phase 0.5 of the reflected light from the unit FBGs is shifted by a half wavelength, namely 0.5λ, where λ represents the wavelength of an optical pulse. In terms of an angular phase difference, it is shifted by (2π×0.5). By setting the wavelength of an optical pulse to be encoded and decoded to λ, the phase difference between the relative phase 0 of the Bragg-reflected light from the unit FBG and the relative phase 0.5 of the Bragg-reflected light from the unit FBG is equal to 0.5λ.
Taking into account of the nature of the phase described above, a relative phase, when taking a specific numerical value, will be represented in the following manner. For instance, the relative phase having a value 0 of the Bragg-reflected light from a unit FBG is simply denoted as 0 (or zero) and the relative phase having a value 0.5 of the Bragg-reflected light from a unit FBG is as 0.5, with the constant part λ or 2π thus omitted.
With regard to the reference unit FBG, it is useful to describe a unit FBG placed on either end of the SSFBG. Thus, as for a reference unit FBG, a unit FBG placed on either the input/output end or on the end opposite to the input/output end of the SSFBG will be described, but no reference will be made to which unit FBG is chosen as the reference unit FBG. As a reference unit FBG, selected is one of the unit FBGs having the relative phase of the Bragg-reflected light denoted as zero in the figure and provided on the opposite ends, which one unit FBG is either the input/output end or the end opposite to the input/output end of the SSFBG.
Now, a detailed description will be made about the phase of the Bragg-reflected light. The shape of an optical pulse observed as a time-serial waveform exhibits a time-serial waveform of the envelope of an optical carrier wave forming the optical pulse. Accordingly, the phase of Bragg-reflected light represents the phase of an optical carrier wave that forms an optical pulse reflected by a unit FBG.
An encoder spreads on the time axis an optical pulse having entered its SSFBG comprising a plurality (t) of unit FBGs into t optical pulses aligned on the time axis to be sent out. The spread of the optical pulse on the time axis will simply be called as time spread. As a result of the time spread, the optical pulse entering the SSFBG is encoded and then output. The t optical pulses resultant from time spreading the input optical pulse by the encoder is referred to as chip pulses.
The position of the peak of the chip pulses on the time axis is dependent on the location of the unit FBGs in the SSFBG that have generated respective chip pulses. Furthermore, the phase of the optical carrier waves that form the respective chip pulses is dependent upon the relative position of the periodic refractive index structure of each unit FBG in the SSFBG.
As shown in FIG. 1, the input optical pulse is supplied to the SSFBG of the encoder via an optical circulator which is denoted by an circle on the transmission end. The encoder spreads the received optical pulse in time and transmits it as a train of chip pulses through the optical circulator. In the SSFBG of the encoder shown in FIG. 1, seven unit FBGs are arranged in the wave-propagating direction of an optical fiber. Thus, the number of chip pulses aligned on the time axis and output by the SSFBG in the encoder is seven in this case.
In the following specific example, the SSFBG of the encoder is given seven bits of binary codes (0, 0, 1, 0, 1, 1, 1) representing a pattern of pseudo-random numbers (PNs) in an M-series. The number of terms in a numerical sequence consisting of 0's and 1's and providing codes may be called as code length. In this example, the code length is seven. Furthermore, the numerical sequence providing the codes may be called as code series, and the terms 0's and 1's in the code series may be referred to as chips. The values 0 and 1 per se may be referred to as code value.
The seven unit FBGs in the SSFBG of the encoder respectively correspond to the first chip 0, second chip 0, third chip 1, fourth chip 0, fifth chip 1, sixth chip 1 and seventh chip 1 of the above optical phase codes. The code value 0 or 1 is determined on the basis of the relative phase set in the respective unit FBGs.
When an optical pulse enters the SSFBG of the encoder, the pulse is reflected by the seven unit FBGs and is sent out as Bragg-reflected light a, b, c, d, e, f and g. In this case, the relative phases of the Bragg-reflected light a, b and d are value 0, whereas the relative phases of the Bragg-reflected light c, e, f and g are value 0.5. The train of chip pulses consisting of the Bragg-reflected light having the above relative phases is indicated by a sequence of relative phase values (0, 0, 0.5, 0, 0.5, 0.5, 0.5) which also represent the appearance order of the chip pulses. A chip whose code value is 0 corresponds to a chip pulse having a relative phase of 0, while a chip whose code value is 0.5 corresponds to a chip pulse having a relative phase of 1.
The first and second chips from the optical circulator have the same code values of 0. That is, the phase of the light reflected by the unit FBG corresponding to the first chip is equal to the phase of the light reflected by the unit FBG corresponding to the second chip. Furthermore, since the code value of the second chip is 0 while the code value of the third chip is 1, both chips are different from one another. In other words, the light reflected by the unit FBG corresponding to the second chip and the light reflected by the unit FBG corresponding to the third chip have a phase difference π.
Similarly, as the third chip has the code value of 1 and the fourth chip has the code value of 0, these chips have different codes. Thus, the light reflected by the unit FBG corresponding to the third chip and the unit FBG corresponding to the fourth chip also have a phase difference π. Phase differences with respect to the fifth and remaining chips are the same as above and therefore a description thereon will be omitted.
In this way, the encoder generates the Bragg-reflected light based on the single incoming optical pulse by means of the seven unit FBGs, and then time-spreads the seven reflected light into an encoded chip pulse train. In FIG. 1, a train of seven optical pulses aligned on the time axis at a particular interval depending on the arrangement of the unit FBGs are illustrated above an optical fiber transmission path which connects the transmitting and receiving ends. As is clear from FIG. 1, a chip pulse train is an optical pulse train obtained by spreading in time an optical pulse entering the encoder into a plurality of optical pulses on the time axis. The individual optical pulses spread and arranged on the time axis correspond to the respective chip pulse.
As described, the relative phase of the chip pulses are (0, 0, 0.5, 0, 0.5, 0.5, 0.5). In addition, the Bragg-reflected light a and b have the same phases. Furthermore, the phase differences between the Bragg-reflected light b and c and between the light c and d are equal to π. The same is true in the cases of the reflected light e, f and g. In other words, when the phase of the Bragg-reflected light a is used as a benchmark, the phases of the reflected light a, b and d are equal to each other, whereas the phases of the reflected light c, e, f and g are different from the former ones by π. The train of chip pulses output from the optical circulator on the transmitting end is passed on the optical fiber transmission path and input into the SSFBG of the decoder via the optical circulator on the receiving end.
The SSFBG in the decoder has the same configuration as the SSFBG in the encoder except that the unit FBGs of the SSFBG in the decoder are placed reversely with respect to the optical circulator. It is clear from the figure in which the relative phase values (0.5, 0.5, 0.5, 0, 0.5, 0, 0) are presented in sequence starting from the input/output end of the SSFBG of the decoder that is the reverse order of the relative phase sequence (0, 0, 0.5, 0, 0.5, 0.5, 0.5) in the encoder.
The SSFBG of the decoder is supplied with, as shown in FIGS. 2A and 2B, the train of chip pulses a to g from the SSFBG of the encoder to generate an autocorrelation wave. FIG. 2A illustrates the temporal waveforms of seven chip pulses h to n which are produced through Bragg reflection performed by the seven unit FBGs in the decoder. In this figure, the horizontal axis denotes the time and indicates the progress of time depicted as time points T1 to T13 for descriptive purpose. The time denoted with a smaller numerical value represents the prior time.
A chip pulse train entering the SSFBG of the decoder is reflected by the unit FBG closest to the optical circulator. The light reflected by this unit FBG is referred to as Bragg-reflected light h. Likewise, the light reflected by the subsequent unit FBGs is referred to as Bragg-reflected light j, k, l, m and n.
The chip pulses a, b, c, d, e, f and g contained in the chip pulse train are reflected by the unit FBG closest to the optical circulator in the decoder so as to be aligned on the time axis as indicated by the symbol h in FIG. 2A. The input chip pulse a is an optical pulse with its peak at time T1 on the time axis. The chip pulse b reflected by the unit FBG closest to the circulator is an optical pulse having its peak at time T2 on the time axis. Similarly, the chip pulses c, d, e, f and g reflected by the unit FBG closest to the circulator are optical pulses with their peaks at times T3, T4, T5, T6 and T7 on the time axis, respectively.
The chip pulses a, b, c, d, e, f and gin the chip pulse train are also reflected by the unit FBG, which is second closest to the optical circulator, so as to be aligned on the time axis as indicated by the symbol i in FIG. 2A. The Bragg-reflected light i from this unit FBG has the same phase as the reflected light h. Hence, the optical pulse train denoted on the time axis as the reflected light h has the same phase as the optical pulse train denoted on the time axis as the reflected light i.
Likewise, the chip pulses in the chip pulse train are reflected by the third and fourth unit FBGs from the optical circulator to thereby be aligned on the time axis as indicated by the symbols j and l in FIG. 2A. The phase relationships between the Bragg-reflected light h and the Bragg-reflected light j and l output from the third and fifth unit FBGs are identical. Therefore, the reflected light j and l line up on the time axis in FIG. 2A as the optical pulse trains indicated by the symbols j and l. Although the optical pulses associated with the Bragg-reflected light i, j, k and l shift on the time axis in parallel, the phase relationship between the optical pulses relevant to the reflect light is mutually equivalent.
By contrast, the Bragg-reflected light output by the fourth, sixth and seventh unit FBGs from the optical circulator has the phase difference π with respect to the reflected light by the first to third and fifth unit FBGs. Consequently, the relative phase sequence of the chip pulses in the chip pulse trains h, i, j and l is (0.5, 0.5, 0, 0.5, 0, 0, 0), whereas the relative phase sequence of the chip pulses in the chip pulse trains k, m and n is (0, 0, 0.5, 0, 0.5, 0.5, 0.5). Note that the sequence of the relative phases of the chip pulses in the chip pulse trains is indicated by sequentially presenting the relative phases of the chip pulses from right to left with respect to the time axis shown in FIG. 2A.
FIG. 2B plots an autocorrelation wave of the input optical pulse decoded by the SSFBG in the decoder. The horizontal axis in this figure depicts the time axis corresponding to that in FIG. 2A. The autocorrelation wave is obtained by combining the Bragg-reflected light h to n output from the unit FBGs in the decoder, that is, the autocorrelation wave is the sum of the reflected light h to n shown in FIG. 2A. At time T7 on the time axis shown in FIG. 2B, all optical pulses with the same phases associated with the reflected light h to n are combined, thereby providing the maximum peak of the autocorrelation wave. At other times than time T7 on the time axis in FIG. 2B, not all the seven chip pulses are superimposed and the chip pulses having the same relative phases are not always superimposed.
Consequently, an optical pulse having greater intensity than that of the optical pulse produced at time T7 is not produced at any other times than time T7 on the time axis shown in FIG. 2B. In other words, the optical pulse is spread into the chip pulse train by the SSFBG in the encoder, and the chip pulse train is supplied to the SSFBG in the decoder, so that the decoder generates the autocorrelation wave. In the illustrative example, seven bits, namely the code length equal to seven, of optical phase codes (0, 0, 1, 0, 1, 1, 1) are used, but are not limitative, and the above description will also be effective to other optical phase codes than those described above.
It is interpreted that the autocorrelation wave shown in FIG. 2B is formed via the mechanism described above. The peak waveform in the position of time T1 on the time axis is formed of the Bragg-reflected light h corresponding to the chip pulse a which is reflected by the first unit FBG from the optical circulator in the decoder. Thus, the amplitude of the peak waveform formed at time T1 on the time axis is equal to the amplitude of the chip pulse.
The peak waveform in the position of time T2 is formed as the sum of the Bragg-reflected light i corresponding to the chip pulse b reflected by the second unit FBG in the decoder and the Bragg-reflected light h corresponding to the chip pulse a reflected by the second unit FBG in the decoder. Since the above sum consists of the chip pulses having the same relative phases of 0.5, the amplitude thereof is twice as large as the amplitude of one chip pulse, that is the chip pulse a.
As described, the peak waveforms indicated in the position of times T3 to T13 on the time axis are formed via the same mechanism as the above-mentioned one. The amplitude of the respective peak waveforms varies by factors of 1, 0, 1, 0, 7, 0, 1, 0, 1, 2 and 1 in the temporal order with respect to the amplitude of the chip pulses. The amplitudes of the peak waveforms are indicated by the respective multiple numbers in parentheses in corresponding peak positions as shown in FIG. 2B.
The sum of the amplitudes of the peak waveforms sequentially added up from the peak waveforms at time T1 to time T13 is 1+2+1+0+1+0+7+0+1+0+1+2+1=17. By converting the sum of the amplitudes into an energy equivalent, the energy of the sum of the peak waveforms is 289 times, equal to 172 times, as high as the energy of one chip pulse. That is, the gross energy of the autocorrelation wave is 289 times as high as the energy of one chip pulse. In addition, the amplitude of the peak of the autocorrelation wave is 7 times as large as that of one chip pulse, and therefore it becomes 49 times in terms of energy equivalent. In this way, when the same codes are set in the encoder and decoder, the encoder time-spreads the chip pulse train to supply it to the decoder, and the decoder in turn converts the chip pulse train into the autocorrelation wave.
Next, with reference to FIGS. 3, 4A and 4B, a description will be made on the case where an encoder and a decoder have different codes. The encoder and decoder shown in FIG. 3 have distinctive difference in that the codes set in an SSFBG serving as the encoder on the transmitting end differs from the codes set in an SSFBG serving as the decoder on the receiving end. The other constituent factors than this factor are the same as in FIG. 1.
An optical pulse included in a transmission signal enters the SSFBG of the encoder via an optical circulator. The SSFBG of the encoder is provided with seven unit FBGs arranged sequentially from the end of the SSFBG close to the optical circulator. The relative phases of Bragg-reflected light a, b, c, d, e, f and g output from the unit FBGs are (0.5, 0.5, 0, 0, 0, 0.5, 0.5). The Bragg-reflected light correspond to respective chip pulses.
A train of chip pulses output from the optical circulator on the transmitting end corresponds to the train of chip pulses a, b, c, d, e, f and g shown above an optical fiber transmission path provided between the optical circulators on the transmitting and receiving ends shown in FIG. 3. The chip pulses indicated by the symbols a to g are output in succession from the optical circulator on the transmitting end. The relative phases of the chip pulses are (0.5, 0.5, 0, 0, 0, 0.5, 0.5) in serial order.
The train of chip pulses output from the optical circulator on the transmitting end pass over the optical fiber transmission path and enter the SSFBG of the decoder via the optical circulator on the receiving end. The SSFBG of the decoder is provided with seven unit FBGs arranged sequentially from the end of the SSFBG close to the optical circulator. The relative phases of Bragg-reflected light h to n output from the unit FBGs are (0.5, 0.5, 0.5, 0, 0.5, 0, 0). That is, the codes set in the SSFBG of the decoder differ from the codes set in the SSFBG of the encoder.
FIGS. 4A and 4B show a process in which the train of chip pulses output from the SSFBG of the encoder are decoded by the SSFBG of the decoder to produce a cross-correlation wave. The chip pulse trains h to n represent the temporal waveforms of the optical pulses reflected by the seven unit FBGs. In the figure, the horizontal axis denotes the time axis. The temporal order on the time axis is depicted as times T1 to T13 for descriptive purpose. The time having a smaller numerical value represents the prior time.
The train of chip pulses entering the SSFBG of the decoder is reflected by the first unit FBG that is closest to the optical circulator. The light reflected by this unit FBG is referred to as Bragg-reflected light h. Likewise, the light reflected by the second to seventh unit FBGs is referred to as Bragg-reflected light i to n, respectively.
The chip pulses a to g contained in the chip pulse train are reflected by the unit FBG closest to the optical circulator so as to be aligned on the time axis as indicated by the symbol h shown in FIG. 4A. The chip pulse a reflected by the first unit FBG is an optical pulse with its peak at time T1 on the time axis. The chip pulse b reflected by the first unit FBG is an optical pulse having its peak at time T2 on the time axis. Similarly, the chip pulses c, d, e, f and g are optical pulses with their peaks at times T3, T4, T5, T6 and T7 on the time axis, respectively.
In the second unit FBG from the optical circulator, the chip pulses contained in the train of chip pulses a to g is reflected and aligned on the time axis as indicated by the symbol i shown in FIG. 4A. In FIG. 4A, since the relative phase of each chip pulse in the chip pulse trains indicated by the symbols h to n is defined on the principle described with reference to FIG. 2A, the relative phases are set to 0 or 0.5 as assigned to the respective chip pulses shown in FIG. 4A.
FIG. 4B shows a cross-correlation wave of the input optical pulse decoded by the SSFBG of the decoder. In this figure, the horizontal axis indicates the time axis, of which the scale corresponds to that of the time axis in FIG. 4A. Since the cross-correlation wave is obtained from the Bragg-reflected light h to n output from the unit FBGs of the decoder, it corresponds to the reflected light h to n, shown in FIG. 4A, superimposed on each other. More specifically, the optical pulse is spread into a train of chip pulses by the SSFBG of the encoder, and the chip pulse train is in turn supplied to the SSFBG of the decoder to generate a cross-correlation wave. The mechanism to generate the cross-correlation wave shown in FIG. 4B is the same as the autocorrelation wave shown in FIG. 2B.
In FIG. 4B, the multiples of the amplitude of the peak waveforms comparing to the amplitude of the chip pulses are indicated in parentheses shown above the corresponding peak positions of the peak waveforms. The sum of the amplitudes of the peak waveforms of the cross-correlation wave sequentially added up from the peak waveforms at time T1 to time T13 is 1+2+1+2+3+0+1+2+1+2+1+2+1=19. By converting the sum of the amplitudes into an energy equivalent, it is 361 times, equal to 192 times, as high as the energy of one chip pulse. It means that the gross energy of the cross-correlation wave takes a value 361 times as high as the energy of one chip pulse. Furthermore, the amplitude of the peak of the cross-correlation wave is merely three times as large as that of one chip pulse. That is merely about nine times as high as the energy of one chip pulse in terms of energy equivalent.
The amplitude of the maximum of the peaks forming the cross-correlation wave shown in FIG. 4B is three times as large in magnitude as the amplitude of the chip pulse. On the receiving end, the autocorrelation wave is distinguished from the cross-correlation wave based on the intensity ratio between the peaks forming the autocorrelation and cross-correlation waves. The amplitude of the maximum peak in the autocorrelation wave shown in FIG. 2B is seven times as large in magnitude as the amplitude of the input chip pulse. Furthermore, the amplitude of the maximum of the peaks forming the cross-correlation wave shown in FIG. 4B is three times larger than that of the input chip pulse. Thus, the peak intensity ratio between them is 72:32=49:9.
As described, the gross energy of the autocorrelation wave is 289 times and the gross energy of the cross-correlation wave is 361 times as high as the energy of one chip pulse, thus there being almost no difference in order of gross energy between both waves. In general in conventional encoding and decoding, the gross energy is comparative between an autocorrelation wave and a cross-correlation wave. As a consequence, when the number of channels to be multiplexed increases in the OCDM, the energy of the autocorrelation wave component becomes smaller than that of the cross-correlation wave component.
By way of example, when two channels are to be multiplexed, the ratio of the energy partitioned into the components of autocorrelation and cross-correlation waves, which corresponds to an S/N ratio of a signal, is about 1:1. However, when four channels are to be multiplexed, the ratio of the energy partitioned into the components of the autocorrelation and cross-correlation waves is 1:3.
This is because the autocorrelation wave is generated for one multiplexed channel, whereas the cross-correlation wave is generated for the three channels other than the channel reproduced as the autocorrelation wave. That is, the S/N ratio becomes smaller as the number of channels to be multiplexed increases in the OCDM. For this reason, the receiving end in the OCDM has to perform time gate processing or utilize a nonlinear optical device, or requires special means. Such special means are disclosed in Wei Cong et al., “An Error-Free 100 Gb/s Time-Slotted SPECTS O-CDMA Network Testbed”, Paper Th. 1.4.6, Vol. 3, ECOC 2005.
As described above, an optical multiplex transmitting system typified by the conventional OCDM for transmitting and receiving a transmitting optical pulse signal by time-spreading with codes or the equivalent and multiplexing the signal required special means for overcoming the problem of the decrease of the S/N ratio due to the increase in number of channels to be multiplexed. As a consequence, a conventional apparatus comprised a lot of constituent elements, resulting in complication in structure of the apparatus and also in cost escalation.